Recently, with a need for highly sophisticated information signal processing, an integrated circuit capable of processing signals in a wider range is required. Particularly, in an optical communication system, the transfer rate is remarkably increased, and transfer rates of 2.4 gigabits per second (Gbps) and 10 Gbps are practically used. Besides, research and development have been intensively carried out for a transfer rate equal to or more than 40 Gbps. Since the signals multiplexed in such an optical communication system have a frequency component ranging from several tens of kiloherz (kHz) to several tens of gigaherz (GHz), a signal amplifier for use with a transceiver is required to have a wide-band and flat gain from several tens of kiloherz to several tens of gigaherz.
Furthermore, a circuit to drive an external modulator used in a transmitter and a circuit to directly drive a laser diode in an optical system require a maximum amplitude of six volt, and a high power output as well as aforementioned wide-band and flat gain characteristic are needed. As a wide-band signal amplifier, a traveling-wave amplifier (traveling-wave amplifier circuit) as shown in FIG. 1 has been reported (A Monolithic Gas 1-13 GHz Traveling-Wave Amplifier IEEE Trans., Vol. MTT-30, No. 7, July 1982, pp. 976-981). In FIG. 1, the numeral 51 denotes an input terminal, 52 denotes an output terminal, 57 denotes a field-effect transistor (hereinafter referred to as “FET”), 58 denotes an input terminal of the FET 57, 59 denotes an output terminal of the FET 57, 60 denotes a ground terminal of the FET 57, 61 denotes an input-side terminating resistor, 62 denotes an output-side terminating resistor, 63 and 64 denote distributed constant lines, and 65 denotes a phase adjusting line.
In the traveling-wave amplifier configured in this way, each of the distributed constant lines 63 and gate-source capacitance Cgs of each FET 57 arranged adjacent thereto constitute a pseudo distributed constant line having characteristic impedance Zg, and the pseudo distributed constant lines construct an input-side coupling circuit 66 together with the input-side terminating resistor 61. Additionally, drain-source capacitance Cds of each FET 57, each phase adjusting line 65, and each distributed constant line 64 form a pseudo distributed line, and the pseudo distributed lines form an output-side coupling circuit 67 together with the output-side terminating resistor 62.
Next, description will be given of operation of the traveling-wave amplifier of this kind. A signal received via the input terminal 51 propagates through each distributed constant line 63 in a direction to the input-side terminating resistor 61. Most signals propagating as described above are sequentially distributed to the respective FETs 57 to be amplified. On the other hand, unrequired signals not distributed to the FETs 57 are absorbed by the resistor 61. Therefore, the input-side coupling circuit 66 configured as above can in general obtain a good input reflection property in a wide band without using a matching circuit.
On the other hand, the signal received by each FET 57 is amplified according to a gate width of the FET 57 and then propagates through each phase adjusting line 65 and each distributed constant line 64 toward the output terminal 52. Moreover, since the respective propagation paths from the input terminal 51 to the output terminal 52 are selected to have an equal electric length, the signals amplified by the FETs 57 are sequentially combined with each other by the output-side coupling circuit 67 to be delivered to the output terminal 52. Thanks to the configuration of the circuit 67 described above, a good reflection property can be obtained in a wide band as on the input side.
As a general method to increase the power output from the traveling-wave amplifier of this type, there can be considered a method to increase the number of periodically disposed FETs 57 and a method to increase the gate width of each FET 57. Furthermore, the gain G of the traveling-wave amplifier of this type is approximately expressed as follows:
                    G        ≈                                                            g                m                2                            ⁢                              n                2                            ⁢                              Z                g                2                                      4                    ⁢                                    (                              1                -                                                                            a                      g                                        ⁢                                          l                      g                                        ⁢                    n                                    2                                            )                        2                                              [                  Expression          ⁢                                          ⁢          1                ]            wherein, αg is an attenuation constant per unitary length of the gate-side circuit, lg is a gate-side line length per unitary cell of the FET 57, Zg is characteristic impedance of the gate-side line, n is the number of FETs 57.
According to the expression, if the number n of FETs 57 is not increased and an expression below is satisfied, the gain G is not increased.
                    n        >                  l                                    a              g                        ⁢                          l              g                                                          [                  Expression          ⁢                                          ⁢          2                ]            
Therefore, even if the number of FETs 57 is increased, the gain G is not increased, and the power output from the traveling-wave amplifier does not become higher as a result.
Furthermore, when the gate width of each FET 57 is increased, the gate-source capacitor Cgs is increased and the cut-off frequency of the traveling-wave amplifier lowers, and hence a wide-band output cannot be obtained.
To solve the problem of this kind, Japanese Patent Application laid open No. HEI4-145712 has disclosed a traveling-wave amplifier as shown in FIG. 2. In this connection, the same constituent components of FIG. 2 as those of the traveling-wave amplifier shown in FIG. 1 are assigned with the same reference numerals and description thereof will be avoided.
In FIG. 2, the numeral 76 represents an input-side impedance matching circuit, 77 represents an input-side transmission line, 78 represents an output-side transmission line, and 79 represents an output-side impedance matching circuit. The matching circuit 76 is a circuit which matches external impedance (e.g., 50 ohm (Ω)) on an external input line side with a composite line impedance of the traveling-wave amplifier and which outputs therefrom an input signal from the external input line side without reflection of the input signal. The circuit 76 includes, for example, a 900 micrometer (μm) long matching line having 25 Ω. The input-side transmission line 77 is connected to the input-side impedance matching circuit 76.
The transmission line 77 includes a distributed constant line 73 (e.g., an 80 μm long line with a characteristic impedance of 35 Ω) and an input-side terminating resistor 71 (e.g., 8 Ω) and is linked with an FET 57.
The output-side transmission line 78 includes a distributed constant line 74 (e.g., an 80 μm long line with a characteristic impedance of 45 Ω) and an output-side terminating resistor 72 (e.g., 20 Ω) and is coupled with an output-side impedance matching circuit 79.
The matching circuit 79 is a circuit which matches external impedance (e.g., 50 Ω) on the external output line side with composite line impedance on the output side of the traveling-wave amplifier and which includes, for example, a 600 μm long, 25 Ω matching line.
According to an advantage of the traveling-wave amplifier shown in FIG. 2, since the composite line characteristic impedance on each of the input and output sides is set according to the gate width of the FET 57 to be lower than the external impedance, the gate width of the FET 57 which can be added for each unitary transmission length can be increased and it is hence possible to increase the power output of the traveling-wave amplifier.
FIG. 3 is a graph showing frequency characteristics of a gain (S11), input reflection (S21), and output reflection (S22) of the traveling-wave amplifier shown in FIG. 2. As can be seen from FIG. 3, the gain G (S11), the input reflection (S21), and the output reflection (S22) each have good characteristics in a band from about 20 GHz to about 30 GHz.